QoS CoS Flashcards
(27 cards)
What is the purpose of CoS (Class of Service) in QoS?
CoS is a Layer 2 marking found in the 802.1Q tag that classifies traffic for prioritization.
How many bits are used in the CoS field of the 802.1Q tag?
3 bits, allowing for 8 possible values (0–7).
Which CoS value is commonly used for voice traffic?
CoS 5
Where is DSCP marking located in the OSI model?
Layer 3 (IP header)
What DSCP value corresponds to Expedited Forwarding (EF) typically used for voice?
DSCP 46
What is the main difference between CoS and DSCP?
CoS is Layer 2 and uses 3 bits in the VLAN tag; DSCP is Layer 3 and uses 6 bits in the IP header.
What command enables CoS trust on a switch interface?
mls qos trust cos
What command enables global QoS on a Cisco switch?
mls qos
Which QoS model categorizes and marks traffic for differentiated treatment?
DiffServ (Differentiated Services)
Which QoS method drops excess traffic immediately?
Policing
Which QoS method buffers excess traffic for later transmission?
Shaping
What is a trust boundary in QoS?
The point in the network where incoming QoS markings (CoS/DSCP) are either trusted or overwritten.
What is LLQ (Low Latency Queuing) and why is it important in QoS?
LLQ is a queuing mechanism that combines CBWFQ with a strict priority queue for delay-sensitive traffic like voice.
What is CBWFQ (Class-Based Weighted Fair Queuing)?
CBWFQ allows traffic classification into classes with bandwidth guarantees based on weights and class maps.
What is WRED (Weighted Random Early Detection) used for?
WRED is used for congestion avoidance by randomly dropping lower-priority packets before a queue is full, protecting TCP flows.
In QoS, where is traffic shaping typically applied?
Traffic shaping is usually applied on egress interfaces to buffer excess traffic and send it at a controlled rate.
In QoS, where is policing typically applied?
Policing is usually applied at ingress to drop or mark traffic that exceeds configured rate limits.
What is the main difference between traffic shaping and policing?
Shaping buffers excess traffic, while policing drops or marks it immediately.
What is the difference between DiffServ and IntServ QoS models?
DiffServ uses class-based marking and prioritization; IntServ uses per-flow resource reservation (e.g., RSVP).
What does the ‘mls qos trust dscp’ command do?
It enables the switch to trust the DSCP value on incoming packets for QoS decisions.
What mechanism is used to match traffic classes in CBWFQ or policing policies?
Class-maps using criteria like ACLs, DSCP, or NBAR are used to classify traffic.
Which part of the IP header is used for DSCP marking?
The first 6 bits of the 8-bit DS field (formerly ToS byte) are used for DSCP.
What is the purpose of a policy map in QoS?
A policy map defines actions (e.g., marking, shaping, policing) to apply to classified traffic.
What is the purpose of a service policy in QoS?
A service policy is used to apply a policy map to an interface for enforcing QoS.
